The present invention relates to the general field of systems for injecting fuel into a combustion chamber of a gas turbine engine. More particularly, the invention relates to a system for injecting an air/fuel mixture, which system provides multimode fuel injection enabling at least two independent modes to be defined for injecting the air/fuel mixture, depending on predetermined operating speeds of the engine.
In each injection system of a conventional combustion chamber of a gas turbine engine, fuel is injected in single mode manner via a fuel injector. Two air swirlers centered on the fuel injector deliver respective radial flows of air downstream from the point of fuel injection so as to mix the air and fuel that are to be injected into the combustion chamber and then burnt. The flows of air coming from the two swirlers are generally defined by a Venturi interposed between said swirlers, and a bowl mounted downstream therefrom accelerates the flow of the air/fuel mixture towards the combustion chamber.
The air/fuel mixture obtained by such injection systems needs to be optimized in order to light combustion in the combustion chamber, in order to ensure that combustion is stable, in particular at low operating speeds of the engine, and in order to limit the emission of pollution into the atmosphere, in particular when the engine is operating at full throttle. These requirements imply modes of operation that are often mutually incompatible. For example, stability of the combustion flame, which is necessary in particular at low operating speeds of the engine, is encouraged by having an air/fuel mixture that is non-uniform, presenting rich zones in the air/fuel mixture close to lean zones. Conversely, the formation of pollutants such as nitrogen oxides is limited by making combustion take place in a mixture that is lean and uniform.
A single-mode fuel injection system as described above cannot satisfy all of the above-specified operating requirements correctly. Fuel injection in such systems takes place in zones where the mass of air injected is lower, thereby tending to make the air/fuel mixture non-uniform. Furthermore, fuel injection reduced to a single point is optimized for only one or at most two operating speeds of the engine. In particular, when operating at idling speed, such injection systems do not operate properly, which leads to high levels of carbon monoxide emissions.
In order to mitigate those drawbacks, it is known to use combustion chambers having two heads, where the idea is to separate low and high speed combustion by providing the chamber with fuel injectors distributed on a xe2x80x9cpilotxe2x80x9d head and on a xe2x80x9ctakeoffxe2x80x9d head spaced apart from the pilot head both radially and axially. Although that solution would appear to be satisfactory, a combustion chamber having two heads remains difficult to control and expensive, given the duplication of the number of fuel injectors compared with a conventional single-head combustion chamber.
U.S. Pat. No. 5,816,049 also discloses a system for injecting an air/fuel mixture in which fuel injection takes place in multiple manner via orifices provided in a Venturi defining flows of air coming from a radial swirler and from an axial swirler via orifices that open out into the passage for the flow of air coming from the radial swirler. However, the injection system described in that patent also presents drawbacks. The injection orifices are fed with fuel, in particular via a plurality of feed ducts, thereby considerably increasing the risk of fuel coking. In addition, the particular disposition of the fuel injection orifices relative to the air injection leads to significant risks of fuel penetrating into the air injection circuit.
The present invention thus seeks to mitigate such drawbacks by proposing an injection system comprising a multimode system for injecting an air/fuel mixture which enables an air/fuel mixture to be prepared that is optimized both for low speed conditions and for high speed conditions in order to limit polluting emissions. The invention also seeks to provide an injection system that limits the risks of coking and prevents any ingress of fuel into the air feed system.
To this end, the invention provides an injection system for injecting an air/fuel mixture into a combustion chamber of a gas turbine engine, said injection system having a longitudinal axis and comprising fuel injection means interposed between first and second air injection means, said fuel injection means being disposed in an annular internal cavity of a Venturi, said cavity being defined by a substantially axial upstream wall and by a substantially radial downstream wall, said fuel injection means comprising at least a first fuel admission circuit provided with at least one fuel injection orifice, and a plurality of second fuel admission circuits independent from the first fuel admission circuit(s), each being provided with at least one fuel injection orifice so as to define a plurality of independent modes of injecting the air/fuel mixture depending on determined operating speeds of the engine, wherein the fuel injection orifice of the first fuel admission circuit is formed in the upstream wall of the Venturi so as to inject fuel towards the combustion chamber in a general direction that is substantially perpendicular to a flow of air coming from the first air injection means, and wherein the fuel injection orifices of the second fuel admission circuits are formed in the downstream wall of the Venturi so as to inject fuel towards the combustion chamber in a general direction that is substantially perpendicular to a flow of air coming from the second air injection means.
As a result, the injection system makes it possible both to generate an air/fuel mixture that is uniform and lean under high speed conditions in order to limit polluting emissions of nitrogen oxide, and also to create pockets of gas in stoichiometric proportion under low speed conditions in order to guarantee lighting and combustion flame stability in the chamber while still keeping emissions of carbon monoxide down. The air/fuel mixture is injected in multiple modes depending on the operating conditions of engine. The distribution of fuel in the injection system can thus be under complete control as a function of the mass of air introduced by the air injection means. In addition, injecting fuel in directions that are perpendicular to the flows of air coming from the air injection means improves homogenization of the air/fuel mixture.
Advantageously, the fuel injection orifices of the first and second fuel admission circuits are regularly distributed around the longitudinal axis and occupy angular positions that are mutually offset so as to improve homogenization of the mixture.
A single feed duct can feed fuel to the first and second fuel admission circuits, e.g. via a plurality of concentric tubes. Thus, fuel feed takes place via a single duct, thereby limiting the risks of coking and taking advantage of the cooling that is obtained by fuel flowing in the circuits.
Additional air or fuel injection means centered on the longitudinal axis of the injection system advantageously serve to define additional modes of air/fuel mixture injection. Such means are mounted on a bowl centered on the longitudinal axis and extending downstream from the first air injection means.